Please make sure that you have removed the outer petals of the rose also known as guard petals. These can be brown around the edges. Sometimes the roses can be air locked, meaning there may be an airbubble that has been trapped in the stem which prevents the flower receiving water.
A tip is to take the roses out of the vase and cut the stem, place the tip of the cut end in boiling water for 30 seconds. Remove from the water and recut at a 45 degree angle and place in cold water. After half an hour you should notice the roses perk up.